During semiconductor fabrication, integrated circuits are created on a semiconductor wafer (“wafer”) defined from a material such as silicon. To create the integrated circuits on the wafer, it is necessary to fabricate a large number (e.g., millions) of electronic devices such as resistors, diodes, capacitors, and transistors of various types. Fabrication of the electronic devices involves depositing, removing, and implanting materials at precise locations on the wafer. A process called photolithography is commonly used to facilitate deposition, removal, and implantation of materials at precise locations on the wafer.
In the photolithography process, a photoresist material is first deposited onto the wafer. The photoresist material is then exposed to radiation filtered by a reticle. The reticle is generally a glass plate that is patterned with exemplary feature geometries that block some of the radiation from passing through the reticle. After passing through the reticle, the radiation contacts the surface of the photoresist material and changes the chemical composition of the exposed photoresist material. With a positive photoresist material, exposure to the radiation causes the exposed photoresist material to become more soluble in a developing solution. Conversely, with a negative photoresist material, exposure to the radiation causes the exposed photoresist material to become less soluble in the developing solution.
Following exposure to the radiation, the photoresist material is subjected to a developing process. In the developing process, the photoresist material is exposed to the developing solution to dissolve portions of the photoresist material that have been made more soluble by exposure to the radiation. Thus, the more soluble portions of the photoresist material are removed by dissolution in the developing solution, leaving a patterned photoresist layer. It should be appreciated that with sufficient exposure to the developing solution, eventually even portions of the photoresist material that have not been made more soluble by irradiation may partially dissolve in the developing solution. Therefore, it is necessary to control a residence time of the developing solution on the photoresist material such that only portions of the photoresist material that have been modified by irradiation are removed.
Once the photoresist material has been developed to reveal the patterned photoresist layer, the wafer is processed to remove, deposit, or implant materials in the wafer regions not covered by the patterned photoresist layer. Therefore, it should be appreciated that if the patterned photoresist layer is not accurately defined, the wafer regions not covered by the patterned photoresist layer will not be accurately defined, and the subsequent wafer processing using the patterned photoresist layer will be inaccurate. Thus, it is necessary to control the residence time of the developing solution on the wafer to ensure that appropriate portions of the photoresist material are removed during the developing process to provide for an accurately defined patterned photoresist layer.